1. Field of the Invention
The present invention relates to an apparatus and method for simulating the appearance of atmospheric effects such as snow in a computer generated image.
2. Related Art
Simulators are well known which are capable of generating an image on a screen used by for example a trainee pilot. The image is generated from a database of digital information representing for example terrain and buildings as they appear from a viewpoint which may move relative to the terrain. The digital data thus represents a model of a worldspace and the images are representative of the appearance of that worldspace from a particular position. Generally the model will be based on a series of planar polygonal objects of known geometry, position, orientation and surface attributes. Examples of such systems are described for example in U.S. Pat. Nos. 5,363,475 and 5,412,796. The first of these two U.S. patents describes the basic operation of an image generator in terms of dealing with the processing of objects including light points in real time, whereas the second deals with the simulation of atmospheric fog.
The simulation of atmospheric effects such as fog and snow is considered to be of a real significance given the impact that such effects can have on the ability of a pilot to monitor aircraft movements by reference to the view from the aircraft. No realistic simulations of layered fog were available until an implementation of the invention in accordance with U.S. Pat. No. 5,412,796 recently become available. Attempts to provide realistic simulations of snow have to date not been successful. Whereas fog can be represented as effects having the same characteristics over a substantial volume of space, snow by its very nature must be represented as a series of discrete elements, and those elements cannot move in a uniform manner if realism is to be achieved. Whereas in non-real time systems any required level of detail can be produced providing sufficient computing power and time is available, this is not the case in real time systems where the computational resources available to generate a single image are necessarily limited.
One snow simulator has been displayed which represents the appearance of snow falling in the vicinity of an aircraft on the ground. Individual flakes of snow were represented by light points strings of which were generated in a similar manner to that adopted in conventional systems for the simulation of the lights of moving vehicles following each other along a road. Effectively the snow was represented as a large number of light points moving within a limited volume of worldspace within which the simulated aircraft was stationary. Although such a system was successful in providing a realistic image in these limited circumstances, it could not provide a realistic simulation in a sufficiently large volume of worldspace to enable the realistic simulation of the appearance of snow through which an aircraft is moving.
In an attempt to provide a realistic simulation of snow, the above model relying upon light points was developed by relying upon repeating modules of snow, each module including a set of light strings to simulate snow flakes and each module representing a segment of a three-dimensional torus made up by an array of modules. The viewing point was located within a module of the torus, and the torus was moved in translation with the viewing point. The relative movement between individual snow flakes and the viewing point was achieved by rotating the torus about its centre such that the viewing point appeared to move through successive modules. This approach was adopted as it enables relative movement between the torus and the viewing point to take account of both movement of individual snow flakes in worldspace and movement of the viewing point relative to that worldspace. In some circumstances this arrangement provided satisfactory results, for example when flying straight, as in those circumstances a simple rotation of the snow "torus" was able to achieve the correct movement between individual particle of snow and the viewing point. Unfortunately, when the aircraft executed a turn, particularly when moving on the ground where sharp turns are common, it was not possible to realistically represent movement of snow across the field of view represented on the viewing screen of the simulator. The snow model "turned" with the aircraft in a completely unrealistic manner. Thus although some systems relying upon the snow torus model described above were distributed, some users found them unsatisfactory.